In U.S. Pat. No. 5,093,415, a process is provided for producing stereoregular polymers. More particularly, it was found that polymers having an isotactic index of at least 96 percent could be prepared in high yield at high production rates by polymerizing an alpha-olefin in a low pressure gas phase fluidized bed process at temperatures in excess of 50.degree. C. employing a catalyst system comprising (i) a solid catalyst precursor, which includes magnesium, titanium, halogen, and an inside electron donor, i.e., a polycarboxylic acid ester containing two coplanar ester groups attached to adjacent carbon atoms; (ii) a hydrocarbylaluminum cocatalyst; and (iii) an outside electron donor or selectivity control agent, i.e., a silicon compound containing a silicon-oxygen-carbon linkage wherein the atomic ratio of aluminum to silicon is in the range of about 0.5:1 to about 100:1 and the atomic ratio of aluminum to titanium is in the range of about 5:1 to about 300:1.
This catalyst system can be used to provide homopolymers of propylene and copolymers of propylene and one or more alpha-olefins. The high activity at temperatures in the range of about 50.degree. C. to about 110.degree. C. makes this catalyst system and similar catalyst systems using mono- and polycarboxylic acid esters as the inside and outside electron donors such as those mentioned in U.S. Pat. Nos. 4,414,132 and 4,882,380 very attractive; however, in the quest for bimodal or multimodal molecular weight distribution polypropylenes, it is desired to use these catalyst systems in multistage processes. A problem then arises as to how to maintain catalyst activity from stage to stage.
The addition of new catalyst precursor to each stage has been suggested. There is no question that this provides both the requisite catalyst activity and resin productivity in each stage, but, unfortunately, this practice generally leads to the production of a disparate blend of polymer particles, which exhibit poor product performance when compared to the blend produced when the precursor is added only to the first stage, and the first stage catalyst precursor residue is transferred from stage to stage.